# Conditional Probability

#### jordanrs

Hello,

just doing some conditional probability revision and im stuck on the following problems as im just confused about what values to take.

Given that $$\displaystyle P(A) = 0.4, P(B) = 0.6$$ and $$\displaystyle P(B|A) = 0.8$$

Find

$$\displaystyle P(B|\overline {A})$$ and $$\displaystyle P(\overline {A}|\overline {B})$$

Answers from the previous steps are as follows:
$$\displaystyle P(A \cap B) = 0.32$$
$$\displaystyle P(A \cup B) = 0.68$$
$$\displaystyle P(A|B) = 0.533$$

Cheers

#### Anonymous1

$$\displaystyle P(B) = P(B|A)P(A) + P(B|\bar A)P(\bar A)$$

$$\displaystyle \implies P(B|\bar A) = \frac{P(B) - P(B|A)P(A)}{P(\bar A)}$$

$$\displaystyle P(\bar A) = P(\bar A|B)P(B) + P(\bar A|\bar B)P(\bar B)$$

$$\displaystyle \implies P(\bar A|\bar B) = \frac{P(\bar A) - P(\bar A|B)P(B)}{P(\bar B)}$$

#### Soroban

MHF Hall of Honor
Hello, jordanrs!

$$\displaystyle \text{Given: }\;P(A) \,=\,0.4,\;\;P(B) \,=\,0.6,\;\;P(B|A) \,=\, 0.8$$

$$\displaystyle \text{Find: }\;\;(a)\;P\left(B\,|\,\overline {A}\right) \qquad (b)\;P\left(\overline {A}\,|\,\overline {B}\right)$$

Place the information in a chart . . .

. . $$\displaystyle \begin{array}{c||c|c||c|} & P(B) & P\left(\overline{B}\right) & \text{Total} \\ \hline \hline P(A) & 0.32 & & 0.40 \\ \hline \\[-4mm] P\left(\overline{A}\right) & & & \\ \hline \hline \text{Total} & 0.60 & & 1.00 \\ \hline \end{array}$$

Fill in the empty cells:

. . $$\displaystyle \begin{array}{c||c|c||c|} & P(B) & P\left(\overline{B}\right) & \text{Total} \\ \hline \hline P(A) & 0.32 & 0.08 & 0.40 \\ \hline \\[-4mm] P\left(\overline{A}\right) & 0.28 & 0.32 & 0.60 \\ \hline \hline \text{Total} & 0.60 & 0.40 & 1.00 \\ \hline \end{array}$$

$$\displaystyle (a)\;\;P(B\,|\,\overline{A}) \;=\;\frac{P(B \wedge \overline{A})}{P(\overline{A})} \;=\; \frac{0.28}{0.40} \;=\;0.70$$

$$\displaystyle (b)\;\;P(\overline{A}\,|\,\overline{B}) \;=\;\frac{P(\overline{A} \wedge \overline{B})}{P(\overline{B})} \;=\; \frac{0.32}{0.40} \;=\;0.80$$

• Anonymous1

#### Anonymous1

Hello, jordanrs!

Place the information in a chart . . .

. . $$\displaystyle \begin{array}{c||c|c||c|} & P(B) & P\left(\overline{B}\right) & \text{Total} \\ \hline \hline P(A) & 0.32 & & 0.40 \\ \hline \\[-4mm] P\left(\overline{A}\right) & & & \\ \hline \hline \text{Total} & 0.60 & & 1.00 \\ \hline \end{array}$$

Fill in the empty cells:

. . $$\displaystyle \begin{array}{c||c|c||c|} & P(B) & P\left(\overline{B}\right) & \text{Total} \\ \hline \hline P(A) & 0.32 & 0.08 & 0.40 \\ \hline \\[-4mm] P\left(\overline{A}\right) & 0.28 & 0.32 & 0.60 \\ \hline \hline \text{Total} & 0.60 & 0.40 & 1.00 \\ \hline \end{array}$$

$$\displaystyle (a)\;\;P(B\,|\,\overline{A}) \;=\;\frac{P(B \wedge \overline{A})}{P(\overline{A})} \;=\; \frac{0.28}{0.40} \;=\;0.70$$
$$\displaystyle (b)\;\;P(\overline{A}\,|\,\overline{B}) \;=\;\frac{P(\overline{A} \wedge \overline{B})}{P(\overline{B})} \;=\; \frac{0.32}{0.40} \;=\;0.80$$